While the following specifications relate to any optical fiber distribution center, the specifications will utilize the common construction of a passive optical network (PON and Fiber to the Premises networks. However the skilled in the art will readily recognize the applicability of the invention to other optical fiber networks, and the invention extends thereto.
In their most common embodiments, passive optical networks comprise of a fiber connecting a wire center 100 to a branching device (commonly known as a splitter or a splitter coupler 6. These specifications will use the term splitter to encompass all such branching and or branching/coupling devices), and from the splitter 6 to a plurality of other downstream network devices. A simplified example of such network is depicted in FIG. 1, and generally follows ITU (International Telecommunication Union) G.983 standard. A wire center 100 is any point of distribution, such as a service provider voice and data signal collection center, a repeater site, and the like. The wire center is the service provider distribution end, and contains at least one Optical Line Terminal (OLT) 1 which serves to couple data to fibers, that in a typical installation extend for a distance of between 20-40 Km between the wire center and the splitter and thence the user premise location. A trunk fiber 4 connects to a splitter or a splitter/coupler 6, which divides the incoming data stream into a plurality of downstream optical devices such as an Optical Network Terminal (ONT) 5 which are commonly installed in a user premises. Feeder fibers 7 connect the ONT to the splitter. The user premises, splitter, and the wire center are considered remote to each other, however no specific distance is dictated.
A common method for diagnosing and testing optical networks utilizes Optical Time Domain Reflectometry. The reflectometer sends an optical pulse into the network, and analyses returns from the network. Every network is characterized by a ‘signature’ of distribution branch returns. Problems in the network may be analyzed by the nature of the reflections. An example of an optical time domain reflectometer employed for such purposes can be found in U.S. Pat. No. 6,028,661 to Minami et al.
However use of Reflectometry suffers from several shortcomings, two of which are service disruption and a potential for equipment damage. Present Reflectometry techniques call for using a band in which the attenuation is minimized. However this band is also the band which is used for the transmission of data. U.S. Pat. No. 6,396,575 to Holland also calls for use of out of band frequency range, but discloses a band of 1625-1650 nM, to avoid the 1550 nm band. As can be seen in FIG. 3, ITU G.983 calls for PON using a high band of 1550 nM 10b, and two low band ranges of 1490 12b and 1310 nM 9b respectively. The skilled in the art will recognize that those frequencies are used by way of example, and because they are conformant to the ITU G.983 standard, however the invention is applicable to other ranges as well. Data carrying frequencies, either to or from the data center are commonly referred to as payload frequencies.
End equipment such as OLT's use an optical filter 15 to separate the signal to distinct bands. FIG. 3 depict the schematic characteristic of such filters. After being separated by the filter each band is direct to a detector such as a PIN diode based detector (9b, 10b). Such detectors are limited by their signal handling capacities and may be damaged by excessive signal levels.
Testing of a typical PON requires use of high level signals due to a typical high attenuation in areas such as the splitter and the fiber feeders themselves, in addition to the downstream attenuation of the trunk fiber 4. The test signal needs to travel, and thus be attenuated, in both directions, and still be detected in usable levels for analysis. Such high signal levels can oftentimes damage the detectors at the user premises. Thus, utilization of the test band of 1625-1650 nM as proposed for example by Holland may cause damage to the end equipment.
Regular in-service testing of the network offers advantages to network operators by providing the ability to allow single dispatch for cost effective speedy repair breakdowns, and by being able to identify and address problems before they become critical. Therefore there is a clear and as of yet unresolved need for a system that will offer such solution.